In a monolithic SAW convolver having a structure (piezo-electric layer/semiconductor) or (piezo-electric layer/insulator/semiconductor) or an air gap type SAW convolver having a structure (piezo-electric layer/semiconductor) or (piezo-electric layer/air/insulator/semiconductor) the convolution efficiency (hereinbelow abbreviated to F.sub.T) depends generally on the bias voltage applied to the gate electrode. FIG. 4 shows an example of the relationship between the bias voltage V.sub.B and F.sub.T, the capacity between the gate electrode and the earth C, as well as the conductance G in an SAW convolver having a structure (ZnO/SiO.sub.2 /Si).
In FIG. 4 it is shown that F.sub.T has a maximum value at a bias voltage V.sub.OP. This bias voltage V.sub.OP is the optimum bias for the convolver and it is desirable that the convolver is driven always at this bias.
However, when a bias is applied to the convolver, in many cases, interfacial energy levels at the interface of semiconductor/insulator, traps at the interface of insulator/piezo-electric layer and traps in the piezo-electric layer can capture or generate electrons and holes and a fairly long time can be required for stabilizing the operation of the convolver because of the time necessary for this capture or generation. Further the relationship between the bias voltage V.sub.B and F.sub.T can depend on the history of the bias voltage applied previously thereto because of the traps and the interfacial energy levels described above. If a bias voltage different from V.sub.OP has been previously applied thereto, the optimum bias voltage thereafter can shift from V.sub.OP. These phenomena are reported in "A Detailed Theory of the Monolithic Zinc Oxide on Silicon Convolver" by B. T. Khuri-Yakub and G. S. Kino, published in IEEE Transaction on Sonics and Ultrasonics, Vol. SU-24, No. Jan. 1, 1977 (USA), p, 34.
Because of the characteristics described above, heretofore a fairly long warming-up times was required after having applied the bias voltage V.sub.OP to the convolver until F.sub.T reaches a satisfactorily great value in order to start the convolver.
Furthermore, since the optimum bias voltage V.sub.OP varies depending on the temperature, by the method by which a constant bias voltage was applied to the convolver, it was difficult to drive it always with the maximum F.sub.T.
For this reason, heretofore the optimum bias voltage V.sub.OP was applied always by means of a battery backup circuit indicated in FIG. 5. In FIG. 5 reference numeral 1 is a convolver; 2, 3 are inputs; 4 is an output gate; 5 is a battery; 6 is a thermistor; 7 is a variable resistor; 8 is an impedance; and 9 is an output. Further it was necessary that the bias voltage by means of this battery should vary so that F.sub.T of the convolver is always greatest. By this method not only the life of the battery gives rise to a problem but also it is difficult to make the gate bias voltage follow completely the variations in V.sub.OP with respect to the variations in the temperature. Therefore it was not possible to avoid worsening in F.sub.T of the convolver with respect to the variations in the temperature.